The proposed research will focus on the development of an analytic technique to differentiate the cognitive changes associated with dementia, depression, and normal aging. This technique is based upon ALLOMETRIC ANALYSIS, an approach commonly used in biology but rarely applied in the area of psychology. Reaction time performance of demented, depressed and healthy age-matched controls will be compared and contrasted using allometric analysis. Just as different allometric relations charcterize different families of biological species, it is hypothesized that different allometric relations characterize people in different diagnostic categories. The utility of this approach will be assessed with respect to two applications: (1) the characterization of groups of subjects in different categories; (2) the diagnosis of individuals based on their reaction time performances. The first application may provide insight into the mechanisms underlying cognitive changes, while the second application may have direct practical implications. A mathematical theroy of general slowing, the INFORMATION LOSS MODEL, will provide the basis for interpreting differences in the parameters of allometric equations and for understanding the processes underlying cognitive changes in depressed, demented, and healthy older adults. The proposed research represents a major shift in the research interests of the Principal Investigator, and an opportunity to apply skills acquired doing basic research to questions with more direct relevance to mental health issues.